1. Field Of The Invention
This invention relates to a battery-powered electronic strobe flash unit and, more particularly, to a simplified energy-regulating circuit for energizing a flashtube of the flash unit, the flashtube being energizable by a voltage that is greater than the battery's voltage.
2. Description of the Prior Art
In order to energize a flashtube of an electronic strobe flash unit, a relatively high voltage, termed the flashtube-breakdown voltage, is required. The breakdown voltage can be reduced when a trigger electrode is provided adjacent the outside of the flashtube, and a voltage pulse, commonly called a flashtube trigger pulse, is applied to the trigger electrode. Nevertheless, a voltage of approximately 300 volts is still required to fire the flashtube.
When the electronic strobe flash unit is powered by a battery having a voltage that is less than the flashtube-breakdown voltage, a DC to DC converter is used to develop the necessary voltage. Typically, a flash-firing capacitor is charged by the converter to store electrical energy used to produce a main flash of light by the flashtube. If a flashtube trigger electrode is provided, often a separate smaller capacitor, commonly called the flashtube trigger capacitor, is also charged by the converter, and provides a flashtube trigger pulse.
The flashtube of an electronic strobe flash unit emits an amount of radiation which is directly proportional to the amount of electrical energy dissipated in the flashtube. When an electronic strobe flash unit is used to illuminate a scene to be photographed, a flash-firing capacitor should be capable of storing enough electrical energy so that the flashtube can illuminate scenes which are at least about ten feet from the camera. This means that the flash-firing capacitor must store about 18 joules of energy to properly expose a photographic film of average light sensitivity.
However, there are strobe flash unit applications in which the amount of flashtube radiation required is much less than that required for a normal photographic flash exposure. One such application involves camera focusing, and the use of electronic strobe flash apparatus for determining the distance from a camera to the subject to be photographed. In this application, an electronic strobe flash unit can be used to transmit a short-duration pulse of radiation. The intensity of radiation reflected from the scene to be photographed is related to scene distance in a known manner. In an application for focusing a camera in which a pulse of IR radiation is transmitted by an electronic strobe flash unit, the electrical energy required to fire the flashtube is on the order of only about 0.1 joule.
A DC to DC converter for use in an electronic strobe flash unit requires a relatively large number of electrical components. The cost of the converter and flash-firing capacitor is a large portion of the overall cost of an electronic strobe flash unit. Furthermore, a converter can require a long interval to charge the capacitor to the voltage necessary to fire the flashtube. Accordingly, a conventional electronic strobe flash unit powered by a low-voltage battery and adapted for illuminating a scene is not ideally suited to an application that requires only a low level of flash radiation.
In addition, a DC to DC converter for use in an electronic strobe flash unit places a heavy load on the flash unit battery. This load can significantly alter the battery voltage, and the voltage to which the flash-firing capacitor is charged. Therefore, the amount of electrical energy to be delivered to the flashtube, and, accordingly the amount of flashtube radiation to be emitted, can vary as a function of battery voltage.
When an electronic strobe flash unit is used to determine subject distance, it is highly desirable that the amount of flash radiation be accurately regulated. In order that the flash illumination does not vary with battery voltage, it is necessary that voltage-monitor apparatus be provided for regulating the flash-firing capacitor voltage. Such apparatus makes an electronic strobe flash unit having a DC to DC converter additionally complex and adds further to the cost of the flash unit.